Sound barrier

ABSTRACT

A sound barrier is disclosed for attenuating noise emanating from a traffic area such as a freeway. The barrier borders the traffic area and incorporates a plurality of spaced-apart Helmholtz chambers, each comprising an elongated chamber having openings therein. The chambers may be of triangular cross section with an edge formed by the apex of the triangular cross section facing toward the traffic area. The openings are positioned in the sides of the triangular chambers; the chambers are positioned so that opposing sides of adjacent chambers form inverse-acting acoustic horns focusing the sound energy from the traffic area toward the openings. The openings also act as side-branch filters to assist in the attenuation of the noise.

United States Patent [191 Hauskins, Jr.

[ May 28, 1974 SOUND BARRIER [75] Inventor: John B. Hauskins, Jr., Phoenix,

Ariz.

[73] Assignee: Engineering Corporation of America, Phoenix, Ariz.

[22] Filed: Oct. 24, 1972 [21] Appl. No.: 300,108

[52] U.S. Cl. 181/33 E, 181/33 R, 181/33 C, 181/33 D, 181/33 L [51] Int. Cl. F0ln 1/10 [58] Field of Search 181/33 G, 33 GE, .5 F

[56] References Cited UNITED STATES PATENTS 3,382,947 5/1968 Biggs 181/33 G Primary Examiner-Stephen .I. Tomsky Assistant Examiner-Vit W. Miska 5 7 ABSTRACT A sound barrier is disclosed for attenuating noise emanating from a traffic area such as a freeway. The barrier borders the traffic area and incorporates a plurality of spaced-apart Helmholtz chambers, each comprising an elongated chamber having openings therein. The chambers may be of triangular cross section with an edge formed by the apex of the triangular cross section facing toward the traffic area. The openings are positioned in the sides of the triangular chambers; the chambers are positioned so that opposing sides of adjacent chambers form inverse-acting acoustic horns focusing the sound energy from the traffic area toward the openings. The openings also act as side-branch filters to assist in the attenuation of the noise.

5 Claims, 8 Drawing Figures PATENTEDMAY 28 1974 I IMHIIEHILIII SOUND BARRIER The present invention pertains to sound barriers, and more particularly, to barriers for use in attenuating noise generated in a traffic area.

The proliferation of freeways as well as the numbers of automobiles utilizing freeways and other traffic areas has rendered the problem of noise pollution considerably more significant. Studies have been made of such traffic noise and the deleterious effects of such noise as indicated by those studies dictate that a scheme for attenuation is necessary. Of course, decreasing the-noise level by reducing the generated noise is a possible solution (reduction in noise level by automotive redesign, etc.), but the immediate problem is the existing noise level and its attenuation.

The present invention may be described by reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram useful in explianing acoustic effects on simple barriers.

FIGS. 2a and 2b are schematic drawings of simple theoretical Helmholtz resonators.

FIG. 3 is a persepective view, partly broken away, of a Helmholtz chamber constructed for use in the sound barrier of the present invention.

FIG. 4 is a pictorial illustration of a sound barrier constructed in accordance with the. teachings of the present invention and placed between a typical traffic area and an adjacent area. FIG. 5 is across-sectional view of a plurality of chambers as used in the sound barrier of the present invention.

FIG. 6 is a schematic diagram useful in describing the operation of an acoustic sidebranch. 1

FIG. 7 is a schematic diagram useful in explaining the optical properties of the sound barrier of the present invention.

A possible approach to shield traffic area noise from adjacent areas is the utilization of a simple wall or plate. Barriers consisting of a plate" having elastic properties and known thickness affect a sound field in two ways:

I. diffraction of sound waves around barrier;

2. reflection and transmission of sound waves.

FIG. I shows the action of these influences on a typical sound wave striking a barrier. As might be expected, the objective in barrier design is to keep the sum of L'" and L as small as possible with respect to L This objective can be achieved with only limited success using conventional earthworkv or solid upright barriers.

The diffracted sound field in the area of the shadow zone of the barrier relative to the sound field in the absence of the barrier determines the effective attenuation of the barrier. Fresnel developed integral equations to determine the attenuation theoretically. However. most treatments of the effect of barrier attenuation in recent years utilize the empirical results obtained by Maekawa, Noise Reduction by Screen," Applied Acoustics, I., (I968) pp. 157 ff, in which he treated the incident sound energy impinging on the barrier surface as behaving in a ray fashion.

Maekawas model has come into widespread use in the design and evaluation of traffic noise barriers. since its use is based on the following generally accepted assumptions:

I. it applies to either line or point sources;

2. the effective wavelength of trafficnoise is about 2 to 3 feet (300 500 Hz); and

3. automobile noise sources are located on or near the road surface, while truck noise sources are effectively between the road surface and the exhaust exit.

It has been suggested that barriers be attributed with a maximum attenuation of 15 dB, due to the influence of diffraction effects over the barrier. With simple barriers, maximum noise level reductions appear to be achievable only at extreme wall heights I 2 feet) and at the higher frequencies l,000 Hz).

The principal disadvantages of conventional barriers for traffic noise attenuation may be summarized as follows: first, effective sound reduction is dependent upon barrier height; second, barrier heights of 25 feet or more (such as would be required to achieve attenuations of 20 dB or more) do not blend aesthetically with the surrounding landscape; third, construction costs for high level barriers (such as earth berms, depressed roadways and concrete walls) are in the range of $50 $500 per running foot; and fourth, the motorist has the impression that he is captured within a tunnel and therefore loses his perspective on distance and speed.

Regarding the effectiveness of increased wall height, it should be noted that reflected noise serves to increase the amplitude of the sound waves traversing the top of the opposite wall. This applies in the case where a barrier is emplaced on both sides of the roadway. The actual noise levels impinging on the barrier surfaces undoubtedly exceed those derived from the empirical models for a given traffic flow pattern. The reader can observe this phenomenon for himself by observing the increase in apparent road noise as his vehicle enters a section of depressed roadway. The reflected sound components from the walls simply reinforce the noise generated at a discrete moment in time. The reflected noise component is partially dissipated in the traffic corridor between the barrier walls (much to the annoyance of the motorist) and partially amplifies the component of diffracted noise over the barrier.

.T he sound barrier of the present invention offers the potential for eliminating the disadvantages of conventional highway noise barriers. This improvement in noise attenuationperformance is attributable to the interaction of several phenomena. Helmholtz resonating chambers are integral to the design of the screen; by varying the length of the cavity opening it is possible to tune the resonator to any selected frequency. Inverseacting acoustic horns focus the sound energy toward the openings of the Helmholtz resonators, thus greatly increasing their efficiency. The cavity openings of the Helmholtz chambers act as side-branch filters for the incident sound waves, adding to the dampening effect on the transmitted component of the sound wave.

The underlying hypothesis for the sound screen is that the net effect of the attenuation phenomena listed above will exceed 25 dB, effectively 10 dB down from the diffracted component of noise reported for conventional solid barriers. Thus, even though the sound screen has a measurable transmitted component of sound, its affect on the overall noise level for an observer will be essentially negligible, and the sound screen will be at least as effective as the conventional barrier.

The simple Helmholtz resonator may be discussed in terms of its analogous mechanical counterpart, the damped spring oscillator. To develop this analogy for illustrative purposes, consider the simple Helmholtz resonator shown in FIG. 2a as consisting of a rigid enclosure of volume V, connected to the external air mass through a small opening of radius a and length L. The gas in the cavity opening corresponds to the spring in the mechanical system and may be considered to move in and out as a unit under fluctuating pressure from the external air. The pressure of the air inside the cavity changes as it is alternately compressed and expanded due to movement of the air in the neck, and this corresponds to the stiffness element of the mechanical analog. Radiation of sound energy into the surrounding air at the neck leads to dissipation of acoustic energy and thus corresponds to the resistance in the mechanical system. There is also a dampening effect due to viscous flow as the air in the neck moves back and forth.

lt will prove useful to determine the effect of the mouth length, L, on the operation of the resonator. Since some of the air beyond the ends of the opening moves as a unit with the air in the mouth, it is necessary to define the true length of the opening as L L 2AL, AL being the additional length on each side of the opening. Then the gas in the opening has a total effective mass of po AL where p is the density of air, A is the cross-sectional area of the opening and L its effective length. Using the analogy of a vibrating piston, mounted in a cylinder of cross-sectional area A, it can be shown that at low frequencies adjacent medium loads cause a vibration of the piston AL 8a/31r. Thus, in our acoustic model: L L 2AL L l6a/31r. For a L, the length of the opening is negligible and a resonator having the configuration shown in FIG. 2!) will also have a definite effective length and volume.

The resonance frequency of the Helmholtz chamber occurs at the frequency too when the acoustic reactancc equals zero. That is, the energy impinging on the resonator is radiated back to the external medium exactly in phase except for some viscous energy losses at the neck due to the large increase in the amplitude component under these conditions.

The resonant frequency for a given Helmholtz chamber can be expressed by the formula woM l/woC 0 where:

M the acoustic inertance of the resonator poL/A C the acoustic compliance of the resonator V/poC c the speed of sound in air at STP.

L neck length of resonator Therefore,

w0= VlilViE? =c- A L V lt should be noted that the shape of the resonator chamber is not a factor in determining the resonant frequency of a Helmholtz chamber. For a given opening, it is the volume of the cavity, not its shape that is important. In fact, as long as the linear dimensions of the cavity are considerably less than a quarter wavelength and the opening is not too large, the resonant frequencies of cavities having the same opening but very different shapes are found to be identical.

lt is of interest that Helmholtz resonators have additional resonant frequencies which are higher than the fundamental frequency me. These frequencies result from. patterns of standing waves in the cavity rather I than the oscillating mass of air at the orifice. Consequently, the overtone frequencies depend on the shape of the cavity, rather than on its volume. This attribute may be of significance in later studies when an attempt is made to increase the frequency range over which damping occurs.

The increase in amplitude at the resonant frequency is denoted by the quality factor,

Q woM/R where:

R the acoustic resistance of the system R poC K /21r and K =wo'poC A /V the effective stiffness of the system.

Therefore, Q 21r L V/A This expression holds true if there are no losses except those resulting'from radiation of sound energy back to the external air. However, in this event the res onator would attenuate the frequency band in close proximity to the resonant frequency and very little attenuation would occur at other frequencies. lf, however, additional damping were introduced into the system, the resistance would be increased due to internal energy losses and the resonator efficiency would be diminished. However, it would attenuate sound levels over a broader frequency spectrum than before the introduction of the damping.

The mathematical basis for the behavior of Helmholtz resonators assumes that the interior cavity is linked to the outside air mass through only one orifice; however, multiple orifices are utilized on both sides of the resonator cavity in the barrier of the present invention. It can be domonstrated that the symmetrically placed orifices effectively provide the same wavelength of incident sound at which resonance occurs.

Referring now to FIGS. 3 and 4, the sound barrier of the present invention is shown in a preferred form. The barrier shown generally at 10 includes a plurality of Helmholtz chambers 12, each comprising a chamber having a constant cross section with an essentially triangular exterior. The chambers incorporate openings or throat areas 15, the numbers and sizes of which are determined in accordance with the frequency range of greatest significance; those frequencies in the 500 Hz. range are considered to be the most appropriate for traffic areas such as highways. The chambers 12 may be secured in place in any convenient manner, such as channel members 16 and 17 positioned on top of and below the respective chambers; the channel members may be secured in any convenient manner to the chambers to form a unitary structure or barrier. The chambers 12 are Helmholtz chambers having an edge 30 formed by the apex of the essentially triangular exterior of cross section; the edge 20 points in the direction of the traffic area, i.e., points to the direction from which the noise is emanating. The barrier 10 is positioned to border the traffic area indicated generally at 25 and to separate the traffic area 25 from an adjacent area 26.

The sides 29, 30, and 31 of the chamber extend vertically between the channels 16 and 17 with the openings 15 passing from the exterior of the chamber to the interior thereof through sides 29 and 30. The side 31 is positioned facing the adjacent area 26; adjacent chambers are spaced from each other to provide certain optical characteristics to be described more fully hereinafter. The sides 29 and 30 of the respective Helmholtz chambers form inverse-acting acoustic horns with the adjacent sides of the adjacent chambers. These sides 29 and 30 may be approximately 2 /2 inches wide, thus rendering a cross section with a triangular outline that provides a wide angle inverse acoustic horn between adjacent chambers. The openings act as side-branch filters for the incident sound waves emanating from the traffic area 25. The dimensions of the Helmholtz chambers 12 may vary and the materials utilized in their construction may be chosen from a wide selection, the characteristics of which may contribute to the effectiveness of the chambers. For example, one embodiment incorporates extruded aluminum chambers 12 having a height of 3', four such chambers being stacked to present a total height between channels 16 and 17 of approximately 12, while the side 31 ofeach channel is approximately 3 inches wide. Channels with these dimensions have been placed with Va inch spaces or apertures therebetween.

The openings 15, as mentioned previously, are chosen with the attenuation frequency range in mind; in the embodiment described here, these openings are approximately 6 inches in length and 0.15 inch wide; twelve of such openings are placed in each of the sides 29 and 30.

The barrier of the present invention uses the exterior walls of the wedge-shaped Helmholtz chambers as a multiple inverse acoustic horn.

The acoustic horn is essentially a transformer, acting more efficiently than the oscillating mass alone because the horn creates a better impedance match between the oscillating element and the external air. As applied to the present invention, this means that high pressures are created in the throat area, causing the vibrating air mass at the neck of the Helmholtz chamber to achieve maximum resonant amplitudes in frequency bands near the resonant frequency of the chamber. The net result of this air coupling" effect is to maximize viscous energy losses for sound waves entering the Helmholtz chamber.

To analyze the pressure amplification function of the acoustic horn in the barrier of the present invention, refer to FIG. 5, wherein a plane wavefront 40 may be seen impinging on the air column at the mouth of the horn. There will be some scattering of energy as the wave enters the constricting area 40 just inside the mouth, but if another wavefront enters the mouth before the first has an opportunity to dissipate, then the first wave will be forced into an area of increasing pres sure until it reaches the opening 43 of the Helmholtz chamber 44. At this point, if the frequency of the incident wave is near the resonant frequency of the chamber, large amplitude vibrations of the air mass at the neck 46 of the chamber are set up and the system radiates the sound energy back to the surrounding air unless damped by friction losses, increased inertance or viscous losses. lnertance can be increased by the case of such sound absorbant materials as fiberglass panels placed against the walls of the chamber.

Since the acoustic horn will increase the level of the radiated portion of the incident wave under certain conditions, it is necessary to select a horn configuration which will retransmit the radiated sound energy inefficiently at the frequencies of interest. The acoustic efficiency of a horn at a given frequency is related to its shape; that is, horns with long air columns and slowly expanding walls transmit low frequencies best, while high frequencies are best transmitted in horns with quickly flaring sides and shorter air columns. The acoustic horn used in the barrier of the present invention possesses a mouth 50 sufficiently broad to capture the lower frequency sound waves of interest, an air column length 51 which will pennit two or more sound pressure fronts to be present in the horn simultaneously, and an inefficient mouth-air column relationship so that radiated sound from the Helmholtz chamber is returned to the surrounding air with minimum amplification.

The barrier of the present invention utilizes the openings 15 (FIG. 3) as acoustic side-branches to absorb sound energy from the incident sound waves. A simple side-branch in acoustics is illustrated in FIG. 6, where:

2,, lnput acoustic impedance of the branch P,, Pressure losses due to branch,

P,, P,, and P, characteristic pressures of the incident transmitted and reflected sound waves, re spectively.

Sound energy traveling through the sound barrier of the present invention can be considered. as traveling through a short pipe of varying cross section, with the Helmholtz resonators acting as branches off the pipe. Lord Rayleigh, in his Theory of Sound, Vol. ll (Dover Publications, New York 1945]), determined that resonant absorbers behaving as branches of a pipe cause strong selective absorption of sound energy.

If the Helmholtz resonator is considered as the branch, with sound energy proceeding from the mouth of the acoustic horn pipe across the neck of the resonator (branch) and then transmitted through an aperture in the screen, the analogy for the present application is clear. Using the designation A the area of the resonator cavity orifice, R the characteristic acoustic resistance of the resonator, and X,, the acoustic reactance of the resonator, then the ratio of the power dissipated in the chamber to that of the incident wave (01,) is given by:

where:

a, ratio of power dissipated in reflected wave to that of incident wave a, ratio of power dissipated in transmitted wave to that of incident wave.

Therefore, maximum dissipation of energy in the Helmholtz resonator can be achieved by adjusting R and X,, such that 01,, is at a maximum. As can be seen, a number of variables affect the magnitude of a and consequently the trade-offs that can be made are quite numerous.

The use of a Helmholtz resonator as a side-branch in the sound barrier of the present invention thus greatly increases the potential sound energy absorption achievable with this approach over the simple barrier.

The subject sound barrier may be considered a series of wedges separated by thin apertures. As shown in FIG. 7, the observer sees only a narrow angle of view through each aperture, denoted by the angle (1), which is a function of the width of the aperture, 41,, and the distance from the aperture to the observer D. it can also be seen that an incremental displacement of the observer along the line of travel Ax (x x, Ax) will have a corresponding effect on the displacement of the angle of view beyond the aperture, which can be denoted as A0. Then, using the instantaneous case Ax dx and A d0, changes in position with time can be expressed as dx/dt K(d0/dt) where K is some constant of proportionality related to the distance D. The term dx/dt is simply the velocity vector along the observers path, which in this case equals the average speed of a passing motorist.

It is of interest in this analysis to determine the range at which an observer traveling at freeway speeds receives overlapping views of the field beyond the screen within the time which the retina of the eye stores an image (approximately 1/ second). By means of a serial strobe effect, the observer sees" a series of views of the field consisting of a number of angles of view of width (r D) sin d), d) being displaced at a rate ofd6/dr. Letting the separation distance between apertures equal b then it is seen that 0 must move through an angular displacement of at least b, a, at some range r from the sound screen within 1/10 second. For D feet, dx/dr 60 miles/hour, ax /8 inches and b, 3 inches, this range r at which the observer apparently sees without obstruction can be approximated from the following expression:

Assuming negligible for this case, the value of r resulting from this calculation is 0.6 foot. Thus, the passing motoristwould effectively see objects behind the screen which were farther than one-half foot away from the apertures.

Some consideration regarding reflected light is necessary to understand the clarity with which the objects behind the screen will be perceived by the observer. The total light energy impinging on the eye of the observer from the direction of the object behind the screen is made up of the following components:

'r n ss TL 0 ER) where:

5,, direct component of light from source (i.e., sunlight) E component of direct light reflected back to observer from front face of sound screen E light reflected from object in direction of ob- I server E component of reflect light from object which is re-reflected from back of sound screen T optical transmission factor of the screen.

From this brief analysis it can be seen that only brightly reflecting objects will be clearly visible behind the sound screen. This problem can be alleviated to some extent by making the front face of the sound screen a poorly reflecting surface, thus increasing the apparent intensity of light received from the object under view. This may, in fact, be an additional advantage to the sound barrier of the present invention in that clear vision is best parallel to the screen (i.e., down the roadway) and while it is possible to see objects through the screen, they will appear slightly subdued in brightness.

I claim:

1. A sound barrier for attenuating noise generated in a traffic area and transmitted to an adjacent area, comprising: a screen bordering said traffic area and positioned between said trafiic area and said adjacent area; said screen including a plurality of vertically extending enclosed chambers each having openings therein to form resonating chambers, and including means for focusing sound energy from said traffic area into said openings; said chambers horizontally separated by optic apertures to present a full view of objects in said adjacent area to moving observers in said traffic area.

2. The combination set forth in claim I, wherein said chambers each have an opening therein positioned with respect to an adjacent chamber to form an acoustic horn for focusing sound energy from said traffic area toward said opening.

3. The combination set forth in claim 1, wherein said resonating chambers are Helmholtz chambers.

4. The combination set forth in claim 2, where each of said chambers is a Helmholtz chamber.

5. A sound barrier for attenuating noise generated in a traffic area and transmitted to an adjacent area, comprising: a screen bordering said traffic area and positioned between said traffic area and said adjacent area; said screen including a plurality of vertically extending Helmholtz chambers each having a constant crosssectional area; each of said chambers including two openings therein, each opening positioned to receive focused sound energy existing between said one of said chambers and an adjacent chamber; said chambers positioned at spaced intervals to intercept and attenuate sound waves emanating from said traffic area.

i: x: a 

1. A sound barrier for attenuating noise generated in a traffic area and transmitted to an adjacent area, comprising: a screen bordering said traffic area and positioned between said traffic area and said adjacent area; said screen including a plurality of vertically extending enclosed chambers each having openings therein to form resonating chambers, and including means for focusing sound energy from said traffic area into said openings; said chambers horizontally separated by optic apertures to present a full view of objects in said adjacent area to moving observers in said traffic area.
 2. The combination set forth in claim 1, wherein said chambers each have an opening therein positioned with respect to an adjacent chamber to form an acoustic horn for focusing sound energy from said traffic area toward said opening.
 3. The combination set forth in claim 1, wherein said resonating chambers are Helmholtz chambers.
 4. The combination set forth in claim 2, where each of said chambers is a Helmholtz chamber.
 5. A sound barrier for attenuating noise generated in a traffic area and transmitted to an adjacent area, comprising: a screen bordering said traffic area and positioned between said traffic area and said adjacent area; said screen including a plurality of vertically extending Helmholtz chambers each having a constant cross-sectional area; each of said chambers including two openings therein, each opening positioned to receive focused sound energy existing between said one of said chambers and an adjacent chamber; said chambers positioned at spaced intervals to intercept and attenuate sound waves emanating from said traffic area. 